The present invention relates to a method of programming nonvolatile memory devices and a multi-level cell (MLC) programming method.
Recently, there has been an increasing demand for nonvolatile memory devices that can be electrically programmed and erased and do not need the refresh function of rewriting data at specific intervals.
The nonvolatile memory cell is an element enabling electrical program/erase operations, and is configured to perform the program and erase operations by changing its threshold voltage as electrons migrate in response to a strong electric field applied to a thin oxide layer of 100 angstrom or less.
The nonvolatile memory device typically includes a memory cell array in which cells for storing data are arranged in matrix form, and a page buffer for writing data into specific cells of the memory cell array or reading data stored in specific cells. The page buffer includes a bit line pair connected to a specific memory cell, a register for temporarily storing data to be written into a memory cell array, or reading data of a specific cell from the memory cell array and temporarily storing the read data therein, a sensing node for sensing the voltage level of a specific bit line or a specific register, and a bit line selection unit for controlling whether or not to connect a specific bit line to the sensing node.
As a method of programming this nonvolatile memory device, an incremental step pulse programming (ISPP) method has been known. That is, a program operation is performed by constantly increasing a program start voltage by a step voltage. However, as the number of program and erase operations increases, the program speed of a memory cell increases. Further, even though a low program start pulse is applied, a change in the threshold voltage of a memory cell increases. If the program start voltage is fixed to a specific value as described above, a change in the threshold voltage becomes great due to the increased program/erase numbers. Consequently, a problem may occur in which threshold voltage distributions are widened.